Schneider Retina lenses in retro position good for APS-C cameras?

As the lens was originally intended to cover a 35mm frame I don't think vignetting will be a problem. There is very little extension involved in such a short adaptor.

You can also reverse mount another lens on to the front of a normally mounted lens, in effect using it as a high quality multi-element close-up lens.

Another possibility is direct reverse mounting of an enlarger lens. These have an M39 mount so you need an adaptor ring in addition to the K mount ring. If you use a lens intended for larger format such as 6x6 you should get full coverage.